Systemic arterial blood pressure measurement provides important diagnostic and health monitoring information, especially for people at risk for hypertension. Blood pressure is also an important measurement in most animal research studies. Intravascular measures of blood pressure, typically via a pressure sensor mounted on a catheter inserted directly into a blood vessel, are considered the “gold standard” for measurement accuracy; however, these intravascular measures require invasive surgery and patient immobilization and cannot be used for simple diagnostic or chronic measurements.
Several methods and techniques have been developed to give physicians, health care workers, and patients themselves the ability to monitor blood pressure. Conventional technology consists of an external pressurized cuff that temporarily occludes an artery in the patient's limb, typically an arm, and means to detect and analyze the Korotkoff sounds or pressure fluctuations as the constriction on the artery is released. Since the auscultatory technique is based on the ability of the human ear or microphone to detect and distinguish sounds, there is a possibility for measurement error due to auditory acuity and sensitivity, outside noise interference, or inconsistent assessment. Other procedures for detecting blood pressure, including oscillometric measurement of vessel pressure against the external cuff, ultrasound, and tonography, are all indirect means of measurement with inaccuracies arising from artifacts, measurement error, patient mobility, or operator consistency, or any combination thereof. Furthermore, proper utilization of the equipment involved in these methodologies requires special training, and each technique is subject to user error.
Implanted sensors provide the most accurate measurement of blood pressure, as the measurement is direct and overcomes the drawbacks of the systems listed above. In the case of severe hypertension, renal insufficiency, or a critical care situation, the benefits of an implanted sensor would warrant the risks of percutaneous techniques. Furthermore, these systems could be implanted in less high risk patient populations when the patients undergo other similar procedures, such as angiography, stent deployment, or balloon angioplasty. Additionally, implantation of these systems in live animal research would provide an accurate and simple means of chronic blood pressure measurement.
Implanted sensors would also allow ambulatory measurements, which provide key insights into diurnal variations in blood pressure, and may provide key information into the underlying disease state and more accurate measurement of a patient's “true” blood pressure (i.e., outside the clinical setting, avoiding the “white coat hypertension” syndrome).
For example, a sensor or transducer placed within a blood vessel or immediately external to the vessel can be used to record variations in blood pressure based on physical changes to a mechanical element within the sensor. This information is then transferred from the sensor to an external device that is capable of translating the data from the sensor into a measurable value that can be displayed. The drawback of this type of sensor is that there must be a physical connection between the sensor and the external device, thus limiting its use to acute settings.
Many types of wireless sensors have been proposed that would allow implantation of the device into the body and then through the appropriate coupling means, so that blood pressure readings can be made over longer periods of interest. One method of manufacturing a sensor capable of measuring pressure is to use a capacitor that is assembled so that the capacitive plates will deform as a result of exposure externally applied stress. This deformation will result in a change in the capacitance that will be proportional to the applied stress. The primary limitation to these type of sensors is that the fabrication methods used to manufacture them do not provide sufficient miniaturization to allow the sensors to be introduced and implanted into an artery using less invasive techniques and the materials used do not provide the appropriate biocompatibility and long term mechanical and electrical durability.
The fabrications methodologies that have been developed in the field of Micro-Electro-Mechanical Systems (MEMS), however, do specifically provide the means for assembling miniaturized sensors capable of measuring a variety of properties including pressure. MEMS devices as described in these patents traditionally use silicon as a substrate for construction of miniature electrical or mechanical structures. The resulting sensors are inherently rigid, severely limiting the ability to manipulate them into temporarily small packages that would provide the means for non-surgical implantation into the human body.
A number of patents detail pressure sensors (some capacitive in nature, some manufactured using MEMS-based technology) that are specifically designed for implantation into the human body. These sensors suffer from many of the limitations already mentioned with the additional concern that they require either the addition of a power source to operate the device or a physical connection to a device capable of translating the sensor output into a meaningful display of a physiologic parameter.
To overcome these two problems (power and physical connection), the concept of an externally modulated LC circuit has been applied to development of implantable pressure sensors. Of a number of patents that describe a sensor design of this nature, Chubbuck, U.S. Pat. No. 6,113,553 is a representative example. The Chubbuck patent demonstrates how a combination of a pressure sensitive capacitor placed in series with an inductor coil provides the basis of a wireless, un-powered pressure sensor that is suitable for implantation into the human body. Construction of an LC circuit in which variations of resonant frequency correlate to changes in measured pressure and which these variations can be detected remotely through the use of electromagnetic coupling are further described in Allen et al., U.S. Pat. No. 6,111,520, incorporated herein by reference.
The device embodied by the Chubbuck patent is manufactured using conventional techniques, thus requiring surgical implantation and thus limiting its applicability to areas that are easily accessible to surgery (e.g., the skull).
Importantly, however, the sensor is not specified as being manufactured using MEMS fabrication technology, and thus no provision is made for appropriate miniaturization of the device that would allow practical and safe introduction and delivery into the body using standard percutaneous approaches.
Thus, there is a need for a method of monitor the systemic arterial blood pressure of living beings in a chronic fashion, such as for the monitoring of severe hypertensive patients or patients at risk for renal failure, or in research studies, where the accuracy of an implanted device is warranted. Furthermore, this method should be accurate, reliable, safe, simple to use, inexpensive to manufacture, convenient to implant and comfortable to the patient.
An ideal method of accomplishing all of the above objectives would be to place a device capable of measuring pressure within or adjacent to an artery. By utilizing an external device to display the pressure being measured by the sensor, a healthcare provider or patient will obtain an immediate readout of blood pressure, which could averaged over time or tracked for diurnal variation.
An example of an implantable pressure sensor designed to monitor blood is shown in Kensey et al, U.S. Pat. No. 6,015,386. While this sensor accomplishes some of the above objectives, it has multiple problems that would make its use impractical. For example, the sensor disclosed in the Kensey patent relies on a mechanical sensing element. Elements of this kind cannot be practically manufactured in dimensions that would allow for endovascular introduction. In addition, this type of pressure sensor would be subject to many problems in use that would limit its accuracy and reliability. One example would be exposure of the mechanical sensing element to body fluids or tissue ingrowth that could disrupt its function. Furthermore, the device fails to account for vascular remolding which would result in baseline drift and could render the device inoperable, as the device requires that the artery be permanently deformed by the clamping action of the sensing element.
Thus, there is a need for a biocompatible, wireless, un-powered pressure sensor that for the purposes of introduction and delivery within the human artery can be manipulated into a smaller shape and size by rolling or folding it into a reduced diameter form and loaded into a small diameter catheter. Then, upon positioning the catheter in the desired location, the sensor can be deployed and secured to the interior of the artery.